This time, Republicans may have to wait till spring — or later — to learn who in their party will challenge President Obama next fall.

In 2008, the GOP presidential nomination battle was effectively over by February, when the revived campaign of Sen. John McCain triumphed over the moneyed war machine of former Massachusetts Gov. Mitt Romney.

But a quick knockout may not be in the cards this time.

Many of the states that jumped to the front of the Republican primary calendar in January and February have had their delegate totals to the nominating convention docked by party officials.

In addition, the 2012 version of Super Tuesday falls on March 6, a month and a day later than its 2008 counterpart. New York and California, both of which held primaries on 2008’s Super Tuesday, won’t weigh in on this cycle until April 24 and June 5, respectively.

With a primary calendar and nominating process that makes it tougher to amass quickly the delegates needed to clinch the nomination, “this year’s schedule will make our nominating process more of a marathon than 2008’s sprint,” said former Hawaii Republican Party Chairman Willes Lee.

Even apart from the structural issues created by the calendar, some Republican analysts also are predicting a long, drawn-out primary based on the specifics of the 2012 candidate field. The poll numbers have shown considerable fluidity, months before Iowa’s first-in-the-nation contest on Jan. 3. There is not a clear, well-liked front-runner, and all of the major candidates have obvious shortcomings and are polling poorly in at least one early state.

“The diverse field, drawn-out process and big differences between the candidate leanings of voters in Iowa, New Hampshire and South Carolina likely mean the nomination won’t be resolved quickly this time around,” said California GOP Chairman Tom Del Beccaro.

Craig Shirley, head of the Washington-based Shirley and Banister Public Affairs and author of biographies of Ronald Reagan, agreed. He called the 2012 nomination race “probably the most wide open since 1940, when Wendell Willkie came out nowhere to seize the chance to challenge the incumbent, Franklin Delano Roosevelt.”

Mr. Shirley and GOP strategists cite the fact that virtually all the top-tier candidates — Mr. Romney, Texas Gov. Rick Perry, businessman Herman Cain and former House Speaker Newt Gingrich — have records that are at least somewhat problematic for conservatives, making it harder for the party base to coalesce quickly around an early consensus choice.

Mr. Gingrich, a star performer in debates who has surged to the top of the Republican polling in recent days, faces questions about his long record and recent revelations of consulting fees he accepted from bailed-out mortgage-financing giant Freddie Mac.

Mr. Cain’s campaign has been slowed by past accusations of sexual harassment and a string of verbal and policy gaffes.

While the steady Mr. Romney has avoided mistakes and run a trouble-free campaign, he has dazzled few in his party’s base and has been unable to build on his consistent national polling numbers — first or second, with the support of about 25 percent of the party.

Rep. Ron Paul of Texas, who has the field’s most devoted following but is equally unloved by others in the party, suddenly finds himself also in the top tier of candidates, scoring 19 percent support in a Bloomberg News poll of Iowa caucus voters, bunched tightly with Mr. Cain, Mr. Romney and Mr. Gingrich.

Mr. Perry was the polling front-runner as soon as he officially entered the race, but several poorly reviewed debate performances have hurt his rankings. Still, GOP campaign professionals say he is likely to remain a factor in the race at least through the early contests next year because he has a deep war chest that he can use for paid advertising.

Former Utah Gov. Jon Huntsman Jr., Rep. Michele Bachmann of Minnesota and former Sen. Rick Santorum of Pennsylvania also remain in the fight, though Mr. Huntsman remains stuck in the low single digits in polls, and Mr. Santorum and Mrs. Bachmann have not been able to build on their isolated victories or minor poll surges.

The crowded field alone is enough to suggest a long haul ahead for the candidates, but some say the crowded field helps Mr. Romney by preventing the forces unhappy with his candidacy from coming together behind a single rival.

“The longer this continues to be a five-plus person race, the more likely it becomes that Romney is the nominee,” said former Virginia GOP Chairman Jeff Frederick. “Should one of the others be able to keep up with Romney in the marathon and benefit from some of the others dropping out, that candidate could be a realistic alternative to Romney, but that may be the only way Romney doesn’t get the nomination.”

Even if candidates drop out, their supporters are not expected to migrate to the same rival candidate, advisers to these campaigns say privately.

Hawaii’s Mr. Lee noted that a long, drawn-out race has become more plausible because of actions by a number of state parties — including in Florida, Arizona and Michigan — to ignore Republican National Committee guidelines and move up their primary dates to early in 2012 in hopes of playing a kingmaker role earlier in the nomination fight.

But jumping the gun will cost them half their regular delegate numbers — and make it harder for a hot candidate to run up the delegate count and drive others from the field, as Mr. McCain did in 2008.

RNC rules also call for subsequent primary and caucus states after the first wave of states to divide their delegates among the candidates instead of adopting the winner-take-all format that Republicans historically have favored.

As a result, it is mathematically impossible for any candidate to accumulate the requisite 1,142 delegates for nomination before March 24, even if that candidate won every vote in every contest from Iowa onward. Unless one candidate emerges to score an early knockout, the earliest practical date anyone can reach the magic number is well into April and possibly later.